Corrosion resistant or acid mist electrostatic precipitators comprise an outer shell or casing which supports an array of tubular collecting electrodes forming a plurality of passages through which the gas streams flow. Located on the center axis of each collecting electrode tube is a corona discharge electrode whose function is to impart an electric charge to the mist. The charged particles are attracted to the collecting electrode. In the case of an acid mist precipitator, at least some of the collection is free-flowing particulate which will flow by gravity down the inner wall of the tube. Auxiliary flushing or flooding may be provided.
For many years, because lead is corrosion resistant, acid mist electrostatic precipitators were fabricated using chemical lead supported by a steel cage. However lead has poor structural properties. It is too fragile to be prefabricated and shipped. Accordingly, the entire lead fabrication has to be done at the site of the construction by highly skilled lead burners. Such construction is expensive. It is also further complicated by the fact that the number of workers in the lead burner trade is rapidly decreasing.
To overcome the disadvantages of lead precipitators, the prior art has proposed that acid mist electrostatic precipitators be constructed from fiberglass reinforced plastic, since such plastic is corrosion resistant. However, such plastic material has a surface resistance of the order of 10.sup.12 ohms and to decrease this undesirably high surface resistance such precipitators have been provided with a continuous flow of liquid intended to provide a thin conductive film over the collecting surface area. However, it has been found to be very difficult to provide a film of wetting liquid uniformly over the total collecting surface. Small streams or rivulets develop leaving dry areas between wet areas and sparkover tends to occur at the boundary between the wet and dry areas. The rise in local temperature due to such sparkover usually results in the destruction of the plastic material. To repair such damage requires a major rebuilding. Attempts have been made to solve this problem by etching the surface of the collecting electrodes but this has not proved satisfactory.
It has also been proposed in the prior art to graphitize the inner surface of the collecting electrode tube or pipe but such conductive surface coatings are washed off in time and require recoatings to avoid destruction by sparkover.
Other proposed coating techniques include painting of the inner surface of the collecting pipe with a conductive varnish or with an epoxy base paint which contains metallic particles, but metallic particles tend to corrode and oxidize which decreases the conductivity, thereby defeating the purpose for which the metallic particles were provided.
The prior art has also proposed admixing graphite particles to fiberglass reinforced polyester resin in such quantity as to bring the surface resistance of the collector down to 10.sup.4 ohms.